HNBR is produced by the heterogeneously or homogeneously catalyzed selective hydrogenation of NBR with hydrogen in an organic solvent.
Little is shown about the recovery of homogeneous hydrogenation catalysts from HNBR solutions. In contrast to heterogeneous hydrogenation catalysts, homogeneous hydrogenation catalysts cannot be removed by simple filtration or centrifugation.
However, the undisputed advantages of homogeneous hydrogenation catalysts lie in their effectiveness in relation to the quantity used and in the fact that they are easy to obtain in constant quality.
For removing a dissolved rhodium complex catalyst from an NBR hydrogenation solution, U.S. Pat. No. 3,700,637 describes precipitation of the rubber from the chlorobenzene/m-cresol solution with methanol and subsequent repeated washing of the coagulated rubber with methanol until the methanol is no longer colored. The noble metal complex may then be obtained from the methanol collected. This is very complicated and very difficult to carry out on a production scale (tons) because, on contact with methanol, the dissolved rubber coagulates into large, tacky viscous lumps which can no longer be readily size-reduced for optimal washing with methanol. In addition, large quantities of solvent have to be handled.
EP-A 354 413 discloses a method for removing soluble Rh catalysts from HNBR solutions with excess, fine-particle organic additives which can adversely contaminate HNBR unless they are carefully removed again.